1. Field of the Invention
The present invention relates to a carrier and a developer for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing, or the like; and an image forming method.
2. Description of the Related Art
In image formation based on an electrophotographic process, a latent electrostatic image is formed on a latent electrostatic image bearing member containing a photoconductive material or the like, charged toner particles are made adhere onto the latent electrostatic image to form a toner image (visible image), and then the toner image is transferred onto a recording medium, such as paper, and fixed on the recording medium to be an output image. In recent years, there have been rapid developments from monochrome image technologies toward full-color image technologies of copiers and printers using electrophotographic processes, and the market of full-color image technologies tends to expand.
Typically, in color image formation based on a full-color electrophotographic process, all colors are reproduced by superimposing three primary color toners of yellow, magenta, and cyan or four color toners with black color toner added to the three primary colors. Therefore, to obtain a full-color image having excellence in color-reproducibility and color vividness, the surface of the fixed toner image must be smoothed and evened to some extent to reduce scattering of light. For this reason, there were so many conventional types of full-color copiers or the like which have a middle level of image glossiness to high level image glossiness of 10% to 50%.
Typically, as a method of fixing a dry toner image on a recording medium, a contact-heating fixing method is heavily used in which a roller or belt having a smooth surface is heated and the surface is press-contacted with a toner. Such a fixing method achieves high thermal efficiency, enables high-speed fixation, and enables imparting glossiness and transparency to color toners, but on the other hand, a so-called “offset phenomenon” is liable to take place in which a part of toner image is attached to a surface of a fixing roller and then transferred onto another image, because it is necessary that the surface of a heat-fixing member be brought in contact with a toner in a melted state under application of pressure and then be separated from the heat-fixing member.
For the purpose of preventing the offset phenomenon, a method is typically employed in which the surface of a fixing roller is formed of a silicone rubber, a fluororesin or the like, which has excellent releasing properties and further, a releasing oil for preventing adhesion of toner, such as a silicone oil, is applied on the surface of the fixing roller. This method is extremely effective in preventing toner offset, however, it requires a device for supplying the releasing oil, and has a problem that the fixing device becomes large in size, and is unsuitable for downsizing of the fixing device. For this reason, in monochrome image formation, there is a tendency to employ an oil-less system where no oil is applied to a fixing roller or a system that requires only a small amount of oil applied to a fixing roller, by using a toner whose viscoelasticity in the melted state is increased by adjusting molecular weight distribution of a binder resin, and which contains a releasing agent such as wax in a toner so that the melted toner does not internally fracture.
Meanwhile, also in full-color image formation, similarly to monochrome image formation, there is a tendency to employ oil-less systems for the purpose of achieving downsizing and structural simplification of fixing devices. In full-color image formation, in order to smooth the surface of a fixed toner image for improving color reproducibility, it is however necessary to reduce the viscoelasticity of toner in the molten state, and thus an offset phenomenon is more likely to occur, as compared to the formation of monochrome images which have no glossiness. Thus, in the full-color image formation, it is difficult to employ oil-less systems in a fixing device and to apply only a small amount of oil. When a toner containing a releasing agent is used, the transferability of the toner onto a recording medium degrades due to increased adhesiveness of the toner. Further, the releasing agent in the toner contaminates a frictional charge member such as a carrier, and the chargeability of the toner degrades, causing a degradation of the durability.
On the other hand, with respect to a carrier, there has been a method in which a hard and high-strength coating layer is formed on a carrier using an appropriate resin material for the purposes of preventing toner component-filming on a surface of carrier, uniformly forming the surface of the carrier, preventing oxidation of the surface of the carrier, preventing reduction in moisture sensitivity, prolonging developer life, preventing the carrier from adhering on surfaces of photoconductors, protecting photoconductors from flaws and frictions which are caused by the carrier, controlling charge polarity, and adjusting the charged amount or the like. There have been carriers proposed so far, for example, a carrier which is coated with a specific resin material (see Japanese Patent Application Laid-Open (JP-A) No. 58-108548); a carrier in which various additives are added to a coating layer (see Japanese Patent Application Laid-Open (JP-A) Nos. 54-155048, 57-40267, 58-108549, 59-166968, and 06-202381, and Japanese Patent Application Publication (JP-B) Nos. 01-019584, and 03-000628); a carrier having additives adhered on the surface thereof (see Japanese Patent Application Laid-Open (JP-A) No. 05-273789); a carrier in which conductive particles each having a diameter greater than the thickness of a coating layer are included in the coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 09-160304), and the like. In addition, Japanese Patent Application Laid-Open No. 08-006307 proposes to use a carrier-coating material which contains a main component of a benzoguanamine-n-butyl alcohol-formaldehyde copolymer. Japanese Patent (JP-B) No. 2683624 proposes to use a cross-linking product of a melamine resin and acrylic resin as a carrier coating material.
However, with these related art approaches, durability and prevention of carrier adhesion are not sufficiently achieved. Further, with respect to durability, there are problems with the amount of toner spent on the surface of carrier, destabilization of the charged amount associated with the toner spent, reduction in the amount of the coating layer caused by abrasion of resin coating film, and reduction in resistance associated with the reduced amount of the coating layer. Therefore, with the carrier according to these related arts, there still remain problems that quality of copied images is degraded as the number of copied sheets increases, although excellent images can be formed in early stage of image formation. A carrier leaves room for improvement.
In response to demands for being faster and more beautiful, high-quality of image, and speeding-up are remarkably progressing in the area of image forming apparatus. In accordance with the movements, the amount of stress that a developer should go through has increased dramatically, and carriers that have been said to be long-lived are unable to have a sufficient operating life. In addition, conventionally, carbon black is frequently used as a resistance controlling agent of carrier, however, there is a fear of smearing a color image with carbon black, and such a smear is caused by exfoliated film of the used resin coating and/or desorption of carbon black. Therefore, various methods have been proposed as a measure to counter the problem and effectively used.
For example, a carrier is formed by making a conductive material (carbon black) reside on the surface of a core material, and the conductive material does not reside in a resin coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 07-140723). The carrier has a concentration gradient of carbon black in the direction of the thickness of a coating layer, in which the concentration of the carbon black is gradually lowered toward the surface of the coating layer, and the carbon black does not reside on the surface of the coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 08-179570). A two-layered coating type carrier is proposed, in which the carrier includes an inside coating layer and a surface coverage layer, the inside coating layer contains a conductive carbon on surfaces of core material particles, and the surface coating layer contains a white conductive material on the inside coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 08-286429). However, these proposals cannot respond to the highly-stressful movements in recent years, and toner smears on images become more and more problematic, and production of carrier leaves room for improvement.
As a drastic measure against such smears on images, it is obvious that non-usage of carbon black which may cause smears on images is the most effective measure. However, when carbon black alone is not used in a carrier, as mentioned above, resistance of the carrier is increased, because carbon black is characterized in its low resistance. Generally, when a carrier having high resistance is used as a developer, with an image having a large surface area of copied image, the image density in the center portion is very low, only edge portions of the copied image are represented with high density, and it is a so-called image that the edge effect is sharply represented. When an image is represented with letters and thin lines, a brilliant image is formed because of the edge effect, however, when an image is represented in neutral color tone, such a carrier has an disadvantage in that images having very poor reproducibility are formed.
Generally, for resistance controlling agents other than carbon black, for example, titanium oxide, zinc oxide or the like are known in the art, however, as to an effect of reducing the resistance, titanium oxide and zinc oxide do not have effect enough to replace carbon black. Accordingly, such resistance controlling agents do not lead to a solution of the problem, and there are still demands for further improvements and developments in carrier.
Also, JP-A No. 11-202560 discloses a carrier whose resistance has been adjusted using an oxide doped with antimony. However, use of antimony is problematic in terms of safety to human and environment. In addition, tin oxide powder containing antimony assumes bluish color, causing the same problem as in carbon black; i.e., the color of the toner is impaired.
Meanwhile, it is conventionally known that a core material is covered with a coating layer(s) containing two different fine particles. For example, JP-A No. 11-184167 discloses a coated carrier including a core material, a first coating layer on the core material, and a second coating layer on the first coating layer, wherein the first coating layer contains needle- or scale-like conductive powder conductive particles and the second coating layer contains conductive particles. JP-A No. 2006-39357 discloses a technique of adjusting resistance using a conductive filler of tin dioxide and indium oxide. The conductive filler has a narrow range within which the resistance can be adjusted, and thus, involves the occurrence of abnormal images (e.g., edge images and halo images) caused depending on the resistance of the carrier. In addition, from the viewpoints of, for example, durability, cost and permanent usability of indium (rare metal), the conductive filler poses various problems and there is room to improve practical use thereof. Under such circumstances, demand has arisen for fine conductive particles which can be controlled in resistance in a wider range, which involves no color staining, and which is made of a material whose reserves are abundant.